Origins
Sunday is the day of the sun, and its name is a sign of both the Roman astronomical system that organized the Western week and the Germanic resistance to Christian renaming that preserved pagan celestial references in everyday speech for over a millennium.
The word derives from Old English 'sunnandæg,' a compound of 'sunnan' (genitive of 'sunne,' sun) and 'dæg' (day). This was a calque of Latin 'sōlis diēs' (day of Sol, the sun), and like Monday (moon's day), it required no mythological substitution through interpretatio germanica. The sun was the sun in both traditions; no Germanic deity needed to be swapped in for a Roman one.
The Proto-Germanic form is reconstructed as *sunnōns dagaz, from *sunnōn (sun), which descends from the PIE root *sóh₂wl̥ (sun). This ancient root is one of the most widely attested in the Indo-European family: Latin 'sōl,' Greek 'hḗlios' (from *sāwelios), Sanskrit 'sūrya' (from *suHriyo-), Lithuanian 'sáulė,' Welsh 'haul,' Gothic 'sauil,' and Old Church Slavonic 'slŭnĭce' all trace back to the same ancestral word. The English words 'solar,' 'solstice,' 'parasol,' and 'insolation' enter the language through Latin, making them distant cousins of the native 'sun.'
Latin Roots
The cultural history of Sunday involves a remarkable tug-of-war between pagan and Christian naming traditions. In 321 CE, the Roman Emperor Constantine issued a decree making 'diēs Sōlis' (day of the Sun) an official day of rest for the empire. This decree — which predated the full Christianization of the Roman state — established Sunday's special status. As Christianity spread, church authorities reinterpreted Sunday as 'diēs Dominica' (the Lord's day), commemorating the resurrection of Christ, which was traditionally placed on a Sunday. Most Romance languages adopted this Christian designation: French 'dimanche,' Spanish 'domingo,' Italian 'domenica,' Portuguese 'domingo,' and Romanian 'duminică' all derive from 'diēs Dominica.'
The Germanic languages, however, resisted this renaming. English 'Sunday,' German 'Sonntag,' Dutch 'zondag,' Swedish 'söndag,' Danish 'søndag,' Norwegian 'søndag,' and Icelandic 'sunnudagur' all preserve the pre-Christian solar reference. This is not because the Germanic peoples were less Christian — by the time these languages were being written down, their speakers were thoroughly Christianized — but because the sun-name was too deeply embedded in daily speech to displace. Church authorities periodically attempted to introduce 'Lord's day' terminology in Germanic languages, and indeed the English phrase 'the Lord's Day' exists as a formal alternative, but it never replaced 'Sunday' in ordinary usage.
The debate over whether Sunday is the first or last day of the week has deep roots. In the original Roman planetary week, Saturday (Saturn's day) was the first day, and Friday (Venus's day) was the last. The Jewish Sabbath falls on Saturday, and the Christian tradition of worshipping on Sunday was established to distinguish the new faith's holy day from the Jewish one. The international standard ISO 8601 designates Monday as the first day of the week, making Sunday the seventh and last day. But in American English and many cultural traditions, Sunday is still considered the first day — a convention reflected in most American calendars, where the leftmost column is Sunday.
Old English Period
In Norse mythology, the sun (Sól) was personified as a goddess who rides a chariot across the sky, pursued by the wolf Sköll who will devour her at Ragnarök. This is one of the few Indo-European traditions where the sun is grammatically and mythologically feminine — in most branches, the sun is masculine (Latin 'sōl' is masculine, Greek 'hḗlios' is masculine). The Germanic feminine sun (Old English 'sunne' is also feminine) is thus a distinctive feature of the northern European branch.
The cultural weight of Sunday in the English-speaking world has shifted dramatically over the centuries. From the strict Sabbatarianism of the Puritans — who enforced Sunday as a day of worship, rest, and moral discipline, with laws against Sunday trading, entertainment, and travel — to the gradual secularization of the day in the twentieth and twenty-first centuries, Sunday has evolved from a day defined by religious obligation to one associated with leisure, brunch, newspapers, and the mild melancholy of the approaching workweek. The phrase 'Sunday best' (one's finest clothes, worn to church) preserves the older religious meaning; 'Sunday driver' (a slow, leisurely motorist) captures the modern secular one.